Tensile Mechanical Behavior and Failure Mechanism of a Plain-Woven SiCf/SiC Composites at Room and Elevated Temperatures

Ceramic matrix composites (CMCs) are the preferred materials for solving advanced aerospace high-temperature structural components;it has the comprehensive advantages of higher temperature (~1500˚C) and low density. In service environments, CMCs exhibit complex damage mechanisms and failure modes, which are affected by constituent materials, meso-architecture and inhere defects. In this paper, the in-plane tensile mechanical behavior of a plain-woven SiCf/SiC composite at room and elevated temperatures was investigated, and the factors affecting the tensile strength of the material were discussed in depth. The results show that the tensile modulus and strength of SiCf/SiC composites at high temperature are lower, but the fracture strain increases and the toughness of the composites is enhanced;the stitching holes significantly weaken the tensile strength of the material, resulting in the material is easy to break at the cross-section with stitching holes.

EFFECT OF FIBER FAILURE ON QUASI-STATIC UNLOADING/RELOADING HYSTERESIS LOOPS OF CERAMIC MATRIX COMPOSITES

The two-parameter Weibull model is used to describe the fiber strength distribution.The stress carried by the intact and fracture fibers on the matrix crack plane during unloading/reloading is determined based on the global load sharing criterion.The axial stress distribution of intact fibers upon unloading and reloading is determined based on the mechanisms of fiber sliding relative to matrix in the interface debonded region.The interface debonded length,unloading interface counter slip length,and reloading interface new slip length are obtained by the fracture mechanics approach.The hysteresis loops corresponding to different stresses considering fiber failure are compared with the cases without considering fiber failure.The effects of fiber characteristic strength and fiber Weibull modulus on the fiber failure,the shape,and the area of the hysteresis loops are analyzed.The predicted quasi-static unloading/reloading hysteresis loops agree well with experimental data.

PROGRESSIVE FAILURE ANALYSIS OF LAMINATED COMPOSITES WITH TRANSVERSE SHEAR EFFECTS

This paper deals with the progressive failure analysis of composite laminates. Triangular elements which include the transverse shear effects are us.d for the stress analysis. A new method for the calculation of the shear correction factors is presented. Several failure criteria are used to check the first ply failure and distinguish the laminate failure modes into fiber breakage or buckling, matrix cracking and delamination. After the failure is detected, the stiffness of the failed ply is modified according to the failure modes. The ultimate strength of the laminate is obtained by an iterative way. Several examples are given in the paper for stress analysis and progressive failure analysis of composite laminates.

Finite element analysis of stress distribution and burst failure of SiC_f/Ti-6Al-4V composite ring

A three-dimensional cyclic symmetry finite element model of titanium-matrix composites（TMCs） ring was developed to investigate the stress distribution and burst failure. The effects of fiber volume fractions, reinforced areas, thermal residual stresses and two different temperatures on stress distribution were studied. The burst speed was obtained through analyzing the hoop tensile stresses under a series of rotating speeds. The results indicate that at the two different temperatures, the influences of fiber volume fractions and reinforced areas on stress level and distribution are different. Some proposals are provided for the structure design of the TMCs ring. With regard to thermal residual stresses, a larger reinforced area is an advisable choice for design of the ring at higher temperature.

Tribological Properties and Failure Analysis of PTFE Composites used for Seals in the Transmission Unit

The sealing rings are one of the most important components as the sealing devices in the wet clutch unit of a heavy vehicle. The sealing ring, made from PTFE composites, was subjected to serious wear on the sealing surface, but the mating metal surface only had slight abrasion. A specialized test rig was designed for wear research and failure analysis of the sealing ring. The composition analyses of the ring material, working conditions and wear surface characteristics by visual inspection and tribological properties as well as microscopic analysis with scanning electron microscope was performed to determine the wear mechanism and failure causes. Results revealed that the wear of PTFE composites was characterized by abrasion and adhesion after a certain duration testing, and the wear mechanism changed to thermal fatigue and abrasive wear in the stage of intense wear. The thermal deformation and fatigue were primarily responsible for the rapid wear of the PTFE composites for the sealing rings.

Hashin Failure Theory Based Damage Assessment Methodology of Composite Tidal Turbine Blades and Implications for the Blade Design

A damage assessment methodology based on the Hashin failure theory for glass fiber reinforced polymer(GFRP)composite blade is proposed. The typical failure mechanisms including the fiber tension/compression and matrix tension/compression are considered to describe the damage behaviors. To give the flapwise and edgewise loading along the blade span, the Blade Element Momentum Theory(BEMT) is adopted. In conjunction with the hydrodynamic analysis, the structural analysis of the composite blade is cooperatively performed with the Hashin damage model. The damage characteristics of the composite blade, under normal and extreme operational conditions,are comparatively analyzed. Numerical results demonstrate that the matrix tension damage is the most significant failure mode which occurs in the mid-span of the blade. The blade internal configurations including the box-beam, Ibeam, left-C beam and right-C beam are compared and analyzed. The GFRP and carbon fiber reinforced polymer(CFRP) are considered and combined. Numerical results show that the I-beam is the best structural type. The structural performance of composite tidal turbine blades could be improved by combining the GFRP and CFRP structure considering the damage and cost-effectiveness synthetically.

Deformation and failure modes of composite foundation with sub-embankment plain concrete piles

With the development of high-speed railway in China, composite foundation with rigid piles has become a stamdard solution of meeting the high requirements of stability and post-construction settlement of embankment on soft subgrade. Among several im- provement pattems, plain concrete piles have been extensively used to treat soft ground supported embankment. To investigate the deformation and failure modes of unimproved soft ground and soft ground reinforced by sub-embankment plain concrete piles, and to learn the influences of track and vehicle load, the effect of pile spacing, as well as the compression moduli of soil layers and upper load condition on the failure modes, a series of centrifuge model tests were performed. Test results indicate that the dis- placement of unimproved soft ground under the embankment increases continuously as embankment, track and train loading, and slip circle failure takes place. The deformation law of soft ground reinforced by sub-embankment plain concrete piles depends on pile spacing, compression modulus of the soft ground, and loading conditions. It was also found that plain concrete piles show displacement and failure patterns depending on its location, compression modulus of soft soil around the pile, and loading condi- tions. Furthermore, the evaluation of improved ground stability as well as the model test procedure is also presented.

NUMERICAL MODELLING OF PROGRESSIVE FAILURE IN PARTICULATE COMPOSITES LIKE SANDSTONE

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Failure analysis and control measures of deep roadway with composite roof:a case study

There is great variation in the lithology and lamination thickness of composite roof in coal-measure strata;thus,the roof is prone to delamination and falling,and it is difficult to control the surrounding rock when developing roadway in such rock strata.In deep mining,the stress environment of surrounding rock is complex,and the mechanical response of the rock mass is different from that of the shallow rock mass.For composite-roof roadway excavated in deep rock mass,the key to safe and efficient production of the mine is ensuring the stability of the roadway.The present paper obtains typical failure characteristics and deformation and failure mechanisms of composite-roof roadway with a buried depth of 650 m at Zhaozhuang Coal Mine(Shanxi Province,China).On the basis of determining a reasonable cross-section shape of the roadway and according to the failure characteristics of the composite roof in different regions,the roof is divided into an unstable layer,metastable layer,and stable layer.The controlled unstable layer and metastable layer are regarded as a small structure while the stable layer is regarded as a large structure.A superimposed coupling support technology of large and small structures with a multi-level prestressed bearing arch formed by strong rebar bolts and highly prestressed cable bolts is put forward.The support technology provides good application results in the field.The study thus provides theoretical support and technical guidance for ground control under similar geological conditions.

Failure criteria for C/SiC composites under plane stress state

The applicability and limitation of several quadratic strength theories were investigated with respect to 2D-C/SiC and 2.5D-C/SiC composites. A kind of damage-based failure criterion, referred to as D-criterion, is proposed for non- linear ceramic composites. Meanwhile, the newly developed criterion is prelim- inarily validated under tension-shear combined loadings. The prediction shows that the failure envelope given by D-criterion is lower than that from Tsai-Hill and Hoffman criteria. This reveals that the damage-based criterion is reasonable for evaluation of damage-dominated failure strength.

Damage Failure Analysis of Z-Pins Reinforced Composite Adhesively Bonded Single-Lap Joint

In order to study themechanical properties of Z-pins reinforced laminated composite single-lap adhesively bonded joint under un-directional static tensile load,damage failure analysis of the joint was carried out bymeans of test and numerical simulation.The failure mode and mechanism of the joint were analyzed by tensile failure experiments.According to the experimental results,the joint exhibits mixed failure,and the ultimate failure is Z-pins pulling out of the adherend.In order to study the failure mechanism of the joint,the finite element method is used to predict the failure strength.The numerical results are in good agreement with the experimental results,and the error is 6.0%,which proves the validity of the numerical model.Through progressive damage failure analysis,it is found that matrix tensile failure of laminate at the edge of Z-pins occurs first,then adhesive layer failure-proceeds at the edge of Z-pins,and finally matrix-fiber shear failure of the laminate takes place.With the increase of load,the matrix-fiber shear failure expands gradually in the X direction,and at the same time,the matrix tensile failure at the hole edge gradually extends in different directions,which is consistent with the experimental results.

Method for Evaluating Bolt Competitive Failure Life Under Composite Excitation

In this study,the competitive failure mechanism of bolt loosening and fatigue is elucidated via competitive failure tests on bolts under composite excitation.Based on the competitive failure mechanism,the mode prediction model and“load ratio-life prediction curve”(ξ-N curve)of the bolt competitive failure are established.Given the poor correlation of theξ-N curve,an evaluation model of the bolt competitive failure life is proposed based on Miner’s linear damage accumulation theory.Based on the force analysis of the thread surface and simulation of the bolt connection under composite excitation,a theoretical equation of the bolt competitive failure life is established to validate the model for evaluating the bolt competitive failure life.The results reveal that the proposed model can accurately predict the competitive failure life of bolts under composite excitation,and thereby,it can provide guidance to engineering applications.

Mechanical properties and failure behavior of 3D printed thermoplastic composites using continuous basalt fiber under high-volume fraction

Continuous basalt fiber(CBF)is an outstanding inorganic fiber produced from nature,which has a wide range of applications in the field of armor protection of national defense military.However,the mechanical response and failure mechanism of 3D printed CBF reinforced components are still not well understood.Here,the 3D printing thermoplastic composites with high volume fraction CBF have been successfully prepared by fused deposition modelling(FDM)method.The effects of fiber printing direction and polymer matrix type on the tensile and flexural properties of the 3D printed composites have been explored,and the detailed failure morphology has been characterized using scanning electron microscopy and optical microscopy.It was found that under high fiber volume fraction,3D printed CBF reinforced polyamides(PA)composites have the best ability to maintain material integrity of the composites,followed by acrylonitrile butadiene styrene(ABS)and high impact polystyrene(HIPS).Besides,the results from rule of mixtures can accurately predict the longitudinal Young’s modulus of the 3D printed specimens,but there exists a large discrepancy for the prediction of the tensile strength.The microstructure analysis shows that the failure modes of 3D printed composites mainly include fiber debonding,fiber pull-out,stress whitening and matrix cracking.

Progressive failure analysis of composite structure based on micro- and macro-mechanics models

Based on parameter design language, a program of progressive failure analysis in composite structures is proposed. In this program, the relationship between macro- and micro-mechanics is established and the macro stress distribution of the composite structure is calculated by commercial finite element software. According to the macro-stress, the damaged point is found and the micro-stress distribution of representative volume element is calculated by finite-volume direct averaging micromechanics(FVDAM). Compared with the results calculated by failure criterion based on macro-stress field(the maximum stress criteria and Hashin criteria) and micro-stress field(Huang model), it is proven that the failure analysis based on macro- and micro-mechanics model is feasible and efficient.

Evaluation of the Reliability Performance of Failure Criteria for Composite Structures

Evolution of materials, following the design requirements of special structures, has shifted interest towards development of composite members able to meet strength requirements “tailored” to specific applications. These members can provide appropriate, more cost effective structures, however absence of generic design guidelines raise constraints towards derivation of optimized structures. Reliability-based assessment can overcome this limitation by ensuring that acceptable levels of target reliability are achieved throughout their service life. This paper presents a methodology for reliability assessment of composite members based on appropriate limit state functions derived according to fundamental failure criteria, Tsai-Hill and Tsai-Wu, applicable to composite materials. The methodology that is proposed employs a Stochastic Response Surface Method (SRSM) which combines in discrete steps FEA modelling, numerical simulations and analytical probabilistic assessment techniques, allowing use of commercial and custom developed specialized numerical tools. Application of the proposed methodology on a complex composite structural geometry will illustrate its efficiency and evaluate the reliability performance of the limit states derived and examined.

Longitudinal Compressive Failure of Multiple-Fiber Model Composites for a Unidirectional Carbon Fiber Reinforced Plastic

The longitudinal compressive failure of a unidirectional carbon fiber reinforced plastic (CFRP) was studied using multiple-fiber model composites. Aligned carbon fibers were embedded in an epoxy matrix and put on a rectangular beam. A compression test of the model composite was performed by means of a four point bending test of the rectangular beam. The number of carbon fibers was changed from one to several thousands, by which the effect on compressive failure modes was investigated. A compressive failure of a single-fiber model composite was fiber crush. The fiber crush strain was much higher than the compressive failure strain of the unidirectional carbon fiber reinforced plastic. By contrast, a compressive failure of a multiple-fiber model composite was kink-band. The longitudinal compressive failure mechanism shifted from fiber crush to kink-band due to an increasing number of fibers. Kink-band parameters i.e. kink-band angle and kink-band width were dependent on the number of closely-aligned carbon fibers.

The Failure Mechanisms of Ultra-high Molecular Weight Polyethylene Fibre Composites Under In-plane Compression

The in-plane compressive characteristics of the ultra-high molecular weight polyethylene(UHMWPE)fibre(Dyneema█)reinforced composites,both in 0/90°and±45°fibre orientations with respect to the loading direction,have been investigated.The composite made from unidirectional high modulus fibres(volume fraction 83%)and low strength polyurethane matrix(volume fraction 17%)is layered in an orthogonally alternating manner.The different failure mechanisms for the composites with 0/90°and±45°fibre orientations have been detected with the methods of experimental measurement,SEM observation and theoretical analysis.The composites specimens of 0/90°fibre orientation failed with macro-buckling of the high-modulus UHMWEP fibre layers with the matrix damage,whereas the specimens of±45°fibre orientation failed with the shearing of the soft matrix.Hence,the composite specimens in 0/90°fibre orientation had higher stiffness as well as compressive strength than those in±45°fibre orientation.The failure criteria of the composites under in-plane compression was employed to characterize the failure mechanism.Compared with the traditional thermoset matrix,the soft thermoplastic matrix leads to lower strength and higher failure strain of fibre reinforced composites under in-plane compression.In addition,the composite specimens cut by waterjet machine exhibited higher stress levels than those cut by bandsaw that introduced more initial imperfections with the temperature rising and tensile shocking.The comparison between the methodologies for cutting the tough composites can provide a valuable suggestion to obtain required composite structures without reducing the mechanical properties.

Prediction of the Biaxial Failure Strength of Composite Laminates with Unit Cell Analytic Model

A new method to predict the ultimate strength of fiber reinforced composites under arbitrary load condition is introduced. The micromechanics strength theory is used to perform the final failure prediction of composite laminates. The theory is based on unit cell analytic model which can provide the ply composite material properties by only using the constituent fiber and matrix properties and the laminate geometric parameters without knowing any experimental information of the laminates. To show that this method is suitable for predicting the strength of composite laminates, the micromechanics strength theory is ranked by comparing it with all the micro-level and the best two macro-level theories chosen from the World Wide Failure Exercise. The results show that this method can be used for predicting strength of any composite laminates and provide a direct reference for composite optimum design.

STUDY ON TENSILE FAILURE MECHANISM AND HYBRID EFFECT OF INTRAPLY HYBRID COMPOSITES

The random critical-core model is adapted to investigate the tensile failuremechanism and hybrid effect of unidirectionally arrayed hybrid composites with alternating low andhigh elongation fibers. By utilizing the model in conjunction with the results of the stressconcentration analysis in which the interfacial damage between fiber and matrix is considered, amicroscopic statistical analysis of both the first failure and ultimate failure of hybrids isperformed. The variations of the first failure strain, the ultimate failure strain and the hybrideffect as the interfacial shear strength are obtained quantitatively. The concept of the hybrideffect for strains has been clarified. The present results are compared with available experimentdata and a reasonable agreement is found between the analytical predictions and the experimentalresults.

Compressive testing and failure analysis of three-dimensionally reinforced carbon/carbon composites

It is important to reveal the performance of carbon/carbon composites subjected to complex loading, which can provide a basis for developing the failure laws of carbon/carbon composites. The uniaxial and biaxial compressive performances of three-dimensional reinforced carbon/carbon composites (3D C/C) were investigated in this paper. The results showed that the compressive strength becomes larger when the loading direction parallels to the z-direction of 3D C/C. The uniaxial compression failure was mainly caused by fracture fiber bundles to form an overall shear fault in the z-direction. The failure mode was delamination of fiber bundle/matrix interface for the x- and y-direction samples. The biaxial compressive failure of x-y direction compressioncompression specimen was caused by the low interlaminar shear strength. In addition,for y-z and z-x direction compression-compression samples,the shear-type failure was formed on the surface of the specimen plumbing the loading direction. Overall,the weak-interface is still a main factor to influent the fracture mechanism of 3D C/C.